59 research outputs found
Depolarization shift of the superradiant phase transition
We investigate the possibility of a Dicke-type superradiant phase transition
of an atomic gas with an extended model which takes into account the
short-range depolarizing interactions between atoms approaching each other as
close as the atomic size scale, which interaction appears in a regularized
electric-dipole picture of the QED of atoms. By using a mean field model, we
find that a critical density does indeed exist, though the atom-atom contact
interaction shifts it to a higher value than it can be obtained from the bare
Dicke-model. We argue that the system, at the critical density, transitions to
the condensed rather than the "superradiant" phase.Comment: 7 pages, 1 figur
Fundamental limitation of ultrastrong coupling between light and atoms
In a recent work of ours [Phys. Rev. Lett. 112, 073601 (2014)], we
generalized the Power-Zineau-Woolley gauge to describe the electrodynamics of
atoms in an arbitrary confined geometry. Here we complement the theory by
proposing a tractable form of the polarization field to represent atomic
material with well-defined intra-atomic potential. The direct electrostatic
dipole-dipole interaction between the atoms is canceled. This theory yields a
suitable framework to determine limitations on the light-matter coupling in
quantum optical models with discernible atoms. We find that the superradiant
criticality is at the border of covalent molecule formation and
crystallization.Comment: 6 page
Atomic selfordering in a ring cavity with counterpropagating pump
The collective dynamics of mobile scatterers and light in optical resonators
generates complex behaviour. For strong transverse illumination a phase
transition from homogeneous to crystalline particle order appears. In contrast,
a gas inside a single-side pumped ring cavity exhibits an instability towards
bunching and collective acceleration called collective atomic recoil lasing
(CARL). We demonstrate that by driving two orthogonally polarized counter
propagating modes of a ring resonator one realises both cases within one
system. The corresponding phase diagram depending on the two pump intensities
exhibits regions in which either a generalized form of self-ordering towards a
travelling density wave with constant centre of mass velocity or a CARL
instability is formed. Controlling the cavity driving then allows to accelerate
or slow down and trap a sufficiently dense beam of linearly polarizable
particles.Comment: 5 page
Elimination of the A-square problem from cavity QED
We generalize the Power-Zineau-Woolley transformation to obtain a canonical
Hamiltonian of cavity quantum electrodynamics for arbitrary geometry of
boundaries. This Hamiltonian is free from the A-square term and the
instantaneous Coulomb interaction between distinct atoms. The single-mode
models of cavity QED (Dicke, Tavis-Cummings, Jaynes-Cummings) are justified by
a term by term mapping to the proposed microscopic Hamiltonian. As one
straightforward consequence, the basis of no-go argumentations concerning the
Dicke phase transition with atoms in electromagnetic fields dissolves.Comment: 5 page
Yielding under compression and the polyamorphic transition in silicon
We investigate the behavior of amorphous silicon under hydrostatic
compression using molecular simulations. During compression, amorphous silicon
undergoes a discontinuous nonequilibrium transition from a low-density to a
high-density structure at a pressure of around -~GPa. Ensemble-averaged
density and elastic constants change discontinuously across the transition.
Densification of individual glassy samples occurs through a series of discrete
plastic events, each of which is accompanied by a vanishing shear modulus. This
is the signature of a series of elastic instabilities, similar to shear
transformation zones observed during shear yielding of glasses. We compare the
structure obtained during compression with a near-equilibrium form of amorphous
silicon obtained by quenching a melt at constant pressure. This gives
structures identical to nonequilibrium compression at low and high pressure,
but the transition between them occurs gradually rather than discontinuously.
Our observations indicate that the polyamorphic transition is of a
nonequilibrium nature, and it has the characteristics of a yield transition
that occurs under compression instead of shear.Comment: 9 pages, 6 figure
Selforganisation and sympathetic cooling of multispecies ensembles in a cavity
We predict concurrent selforganisation and cooling of multispecies ensembles
of laser-illuminated polarisable particles within a high-Q cavity mode.
Resonant collective scattering of laser light into the cavity creates optical
potentials which above a threshold pump power transforms a homogeneous particle
distribution to a crystalline order for all constituents. Adding extra
particles of any mass and temperature always lowers the pump power required for
selfordering and allows to concurrently trap atoms, for which high phase-space
densities are readily available, in combination with many other kind of atoms,
molecules or even polarisable nanoparticles. Collective scattering leads to
energy exchange between the different species without direct collisional
interactions. We analytically calculate the threshold condition, energy fluxes
and the resulting equilibrium phase-space distributions and show that
cavity-mediated energy transfer enhances cooling of heavy particles by adding
light particles forming a cold reservoir. Extensive numerical many-body
simulations support the results of our kinetic analytic model.Comment: 7 pages, 5 figure
Kinetic theory of cavity cooling and self-organisation of a cold gas
We study spatial self-organisation and dynamical phase-space compression of a
dilute cold gas of laser-illuminated polarisable particles in an optical
resonator. Deriving a non-linear Fokker--Planck equation for the particles'
phase-space density allows us to treat arbitrarily large ensembles in the
far-detuning limit and explicitly calculate friction forces, momentum diffusion
and steady-state temperatures. In addition, we calculate the self-organisation
threshold in a self-consistent analytic form. For a homogeneous ensemble below
threshold the cooling rate for fixed laser power is largely independent of the
particle number. Cooling leads to a -Gaussian velocity distribution with a
steady-state temperature determined by the cavity linewidth. Numerical
simulations using large ensembles of particles confirm the analytical threshold
condition for the appearance of an ordered state, where the particles are
trapped in a periodic pattern and can be cooled to temperatures close to a
single vibrational excitation.Comment: 7 pages, 8 figure
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